Zinc-based metal plated steel sheets are widely used in many fields, in particular, for automobile bodies. When used for automobile bodies, they are subjected to press forming before use. Zinc-based metal plated steel sheets, however, have the disadvantage that their press formability is inferior to that of cold-rolled steel sheets. This is because in a press die, the friction resistance of a surface-treated steel sheet is larger than that of a cold-rolled steel sheet. That is, the surface-treated steel sheet does not smoothly flow into the die at a portion of the surface-treated steel sheet having a large friction resistance to the die and a bead. This is liable to cause rupture of the steel sheet.
In recent years, the demand for high-tensile steel sheets has increased to reduce the weight of automobile bodies. High-tensile steel sheets have press formability inferior to that of mild steel sheets. Thus, high-tensile steel sheets are easily ruptured at portions of high-tensile steel sheets having a large friction resistance to dies and beads.
Galvannealed steel sheets are excellent in weldability and paintability compared with galvanized steel sheets and, thus, more preferably used for automobile bodies.
A galvannealed steel sheet is produced as follows: a steel sheet is subjected to galvanizing and then heat treatment. As a result, an alloying reaction in which Fe in the steel sheet and Zn in a plating layer are diffused occurs, thereby forming a Fe—Zn alloy phase. The Fe—Zn alloy phase is in the form of a layer usually including a Γ phase, a δ1 phase, and a ζ phase. Hardness and a melting point tend to decrease as the Fe concentration decreases, i.e., in a sequence of the Γ phase→the δ1 phase→the ζ phase. Thus, a high-hardness, high-melting point film with high Fe concentration is effective from the viewpoint of achieving good tribological properties because adhesion does not easily occur. Galvannealed steel sheets with the emphasis on press formability are produced in such a manner that average Fe concentrations in films are relatively high.
In a film with high Fe concentration, however, hard and brittle Γ phase is readily formed at the interface between the plating film and the steel sheet. Peeling from a surface boundary, i.e., powdering, is disadvantageously liable to occur during processing. Thus, as shown in Japanese Unexamined Patent Application Publication No. 1-319661, for the purpose of striking a balance between tribological properties and anti-powdering properties, a method for forming a hard Fe-based alloy layer as a second layer serving as an upper layer is employed. Disadvantageously, production by the method is costly.
As another method for improving press formability of a zinc-based metal plated steel sheet, a method for applying high-viscosity lubricant oil is widely used. In this method, however, a defect of coating due to a defect of degreasing occurs in an application step because of high viscosity of the lubricant oil. Furthermore, the lack of oil during press forming disadvantageously causes unstable press performance and other problems. Thus, improvement in the press formability of galvannealed steel sheets is strongly required.
As a method to overcome the foregoing problems, Japanese Unexamined Patent Application Publication Nos. 53-60332 and 2-190483 each disclose a technique for improving weldability or processability by subjecting surfaces of a zinc-based metal plated steel sheet to electrolytic treatment, immersion treatment, coating and oxidation treatment, or heat treatment to form an oxide film mainly composed of ZnO.
Japanese Unexamined Patent 4-88196 discloses a technique for improving press formability and chemical conversion treatability by immersing surfaces of a zinc-based metal plated steel sheet in an aqueous solution containing 5 to 60 g/L sodium phosphate and having a pH of 2 to 6, electrolytic treatment, or applying the solution described above to form an oxide film mainly composed of a P oxide.
Japanese Unexamined Patent Application Publication No. 3-191093 discloses a technique for improving press formability and chemical conversion treatability by subjecting surfaces of a zinc-based metal plated steel sheet to electrolytic treatment, immersion treatment, coating, coating and oxidation treatment, or heat treatment to form a Ni oxide.
Japanese Unexamined Patent Application Publication No. 2003-306781 discloses a technique for improving tribological properties by bringing a galvannealed steel sheet into contact with an acidic solution to form an oxide mainly composed of Zn on surfaces of the steel sheet and suppress adhesion between a plating layer and a press die.
The technique for improving press formability by forming an oxide mainly composed of Zn on surfaces of steel sheet disclosed in Japanese Unexamined Patent Application Publication No. 2003-306781 and the like has the advantage over the technique using Ni and the like disclosed in Japanese Unexamined Patent Application Publication No. 3-191093 in production cost and environmental loading because Zn contained in the plated steel sheet is mainly used. In the case where the steel sheet is used for a difficult-to-form component, however, a high degree of press formability is required, so that further improvement in tribological property may be required.
It could therefore be helpful to provide a zinc-based metal plated steel sheet excellent in tribological properties during press forming compared with the technique for improving press formability by forming an oxide mainly composed of Zn on surfaces of a steel sheet.